Ozone generating apparatuses generally include a dielectric electrode and a metal electrode arranged in an airtight container. A spacer for forming a discharge gap is interposed between the dielectric electrode and the metal electrode. A conductive film is arranged on an inner face of the dielectric electrode.
In such an ozone generating apparatus, material gas introduced into the airtight container from a gas inlet flows through the discharge gap formed between the dielectric electrode and the metal electrode. The material gas that has flowed through the discharge gap is then discharged from a gas outlet.
In parallel with the introduction of the material gas, an AC high voltage is applied between the dielectric electrode and the metal electrode from a high-voltage power supply via a fuse and a high-voltage power supply terminal.
When the AC high voltage is applied, dielectric barrier discharge is generated in the discharge gap and ozone is generated. The dielectric barrier discharge may be simply referred to as barrier discharge or silent discharge in some cases.
The heat generated by the dielectric barrier discharge is cooled with cooling water supplied into a cooling water flow path formed with the metal electrode and the airtight container. This configuration can suppress temperature rise of the gas in the discharge gap to efficiently obtain the ozone.
In general ozone generating apparatuses known in the art, a discharge gap length d is assumed to be 0.6 mm to 1.3 mm. A gas pressure p of air serving as the material gas is assumed to be 0.17 to 0.28 MPa (absolute pressure).
A product of the gas pressure p of the material gas and the discharge gap length d is commonly called a pd product. When the pd product is made constant, similarity law of discharge is established. This is because the pd product represents the number of gas molecules in the discharge gap.
For example, multiplication of electrons traveling through the discharge gap is represented by a product αd of an ionization coefficient α of the gas and the discharge gap length d.
The product αd is then rewritten as follows.αd=(α/p)(pd)
where /p represents ionization caused by a single collision, and the pd product represents the number of molecules included in the discharge gap. For that reason, famous Paschen's law that gives a sparkover voltage is a function of the pd product.
The discharge gap length d that has been used is within a region of 0.6 mm or more as described above. In the region in which the discharge gap length d=0.6 mm or more, an optimum value of the pd product with respect to a yield of ozone is a constant value around 20 kPa·cm. Due to this, it has been difficult to further improve the yield of ozone.
Accordingly, the discharge gap length d may be an optimum value of less than 0.6 mm to improve the yield of ozone.
A spacer made of a stainless steel plate (sheet metal spacer) has been used to form the discharge gap.
In view of strength, a thickness of an available ring-shaped sheet metal spacer is about 0.2 mm. For example, when the discharge gap length is assumed to be 0.4 mm, the entire space is substantially filled and is hardly used.
The dielectric electrode is inserted into the sheet metal spacer only at both ends thereof because of its structure. The metal electrode (such as a stainless steel tube) and the dielectric electrode (discharge tube) are both somewhat bent. Due to this, the discharge gap length d does not reach a desired value at a center portion in which the sheet metal spacer cannot be inserted. As a result, ozone generation efficiency (g/kWh) becomes lower than a theoretical value.
The present invention provides an ozone generating apparatus that can keep the discharge gap length constant in a longitudinal direction and achieve higher yield of ozone even when the discharge gap length is less than 0.6 mm.
An ozone generating apparatus according to an embodiment arranges a cylindrical low-voltage electrode coaxially with respect to a cylindrical high-voltage electrode, and applies a predetermined high voltage between the high-voltage electrode and the low-voltage electrode via a dielectric substance to cause discharge for generating ozone.
A discharge gap length d is assumed to be 0.3 mm to 0.5 mm.
Furthermore, any one of the low-voltage electrode and the high-voltage electrode is formed as a metal electrode, and the other one is formed as a dielectric electrode.
A projection group including a plurality of projections is arranged on an inner peripheral surface of the metal electrode opposed to the dielectric electrode for holding the metal electrode to be coaxial with the dielectric electrode while keeping the discharge gap length.